Add Project.RemoveDocuments() as a helper method as well

src/Workspaces/Core/Portable/PublicAPI.Unshipped.txt

 *REMOVED*Microsoft.CodeAnalysis.TextDocument.Project.set -> void
 *REMOVED*Microsoft.CodeAnalysis.TextDocument.TextDocument() -> void
 Microsoft.CodeAnalysis.Options.DocumentOptionSet.WithChangedOption<T>(Microsoft.CodeAnalysis.Options.PerLanguageOption<T> option, T value) -> Microsoft.CodeAnalysis.Options.DocumentOptionSet
+Microsoft.CodeAnalysis.Project.RemoveDocuments(System.Collections.Immutable.ImmutableArray<Microsoft.CodeAnalysis.DocumentId> documentIds) -> Microsoft.CodeAnalysis.Project
 Microsoft.CodeAnalysis.Solution.RemoveDocuments(System.Collections.Immutable.ImmutableArray<Microsoft.CodeAnalysis.DocumentId> documentIds) -> Microsoft.CodeAnalysis.Solution
 Microsoft.CodeAnalysis.Solution.WithOptions(Microsoft.CodeAnalysis.Options.OptionSet options) -> Microsoft.CodeAnalysis.Solution
 static Microsoft.CodeAnalysis.Formatting.Formatter.OrganizeImportsAsync(Microsoft.CodeAnalysis.Document document, System.Threading.CancellationToken cancellationToken = default(System.Threading.CancellationToken)) -> System.Threading.Tasks.Task<Microsoft.CodeAnalysis.Document>

src/Workspaces/Core/Portable/Workspace/Solution/Project.cs

@@ -14,6 +14,7 @@
 using System.Threading.Tasks;
 using Microsoft.CodeAnalysis.Diagnostics;
 using Microsoft.CodeAnalysis.Host;
+using Microsoft.CodeAnalysis.Shared.Extensions;
 using Microsoft.CodeAnalysis.Text;
 using Roslyn.Collections.Immutable;
 using Roslyn.Utilities;
@@ -604,6 +605,23 @@ public Project RemoveDocument(DocumentId documentId)
             return this.Solution.RemoveDocument(documentId).GetProject(this.Id)!;
         }
 
+        /// <summary>
+        /// Creates a new instance of this project updated to no longer include the specified documents.
+        /// </summary>
+        public Project RemoveDocuments(ImmutableArray<DocumentId> documentIds)
+        {
+            foreach (var documentId in documentIds)
+            {
+                // Handling of null entries is handled by Solution.RemoveDocuments.
+                if (documentId?.ProjectId != this.Id)
+                {
+                    throw new ArgumentException(string.Format(WorkspacesResources._0_is_in_a_different_project, documentId));
+                }
+            }
+
+            return this.Solution.RemoveDocuments(documentIds).GetRequiredProject(this.Id);
+        }
+
         /// <summary>
         /// Creates a new instance of this project updated to no longer include the specified additional document.
         /// </summary>

src/Workspaces/Core/Portable/WorkspacesResources.resx

@@ -1440,4 +1440,7 @@ Zero-width positive lookbehind assertions are typically used at the beginning of
   <data name="Document_does_not_support_syntax_trees" xml:space="preserve">
     <value>Document does not support syntax trees</value>
   </data>
+  <data name="_0_is_in_a_different_project" xml:space="preserve">
+    <value>{0} is in a different project.</value>
+  </data>
 </root>
\ No newline at end of file

src/Workspaces/Core/Portable/xlf/WorkspacesResources.cs.xlf

@@ -1317,6 +1317,11 @@ Pozitivní kontrolní výrazy zpětného vyhledávání s nulovou délkou se obv
         <target state="translated">Pracovní prostor není platný.</target>
         <note />
       </trans-unit>
+      <trans-unit id="_0_is_in_a_different_project">
+        <source>{0} is in a different project.</source>
+        <target state="new">{0} is in a different project.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="_0_is_not_part_of_the_workspace">
         <source>'{0}' is not part of the workspace.</source>
         <target state="translated">'{0} není součástí pracovního prostoru.</target>

src/Workspaces/Core/Portable/xlf/WorkspacesResources.de.xlf

@@ -1317,6 +1317,11 @@ Positive Lookbehindassertionen mit Nullbreite werden normalerweise am Anfang reg
         <target state="translated">Arbeitsbereich ist nicht leer.</target>
         <note />
       </trans-unit>
+      <trans-unit id="_0_is_in_a_different_project">
+        <source>{0} is in a different project.</source>
+        <target state="new">{0} is in a different project.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="_0_is_not_part_of_the_workspace">
         <source>'{0}' is not part of the workspace.</source>
         <target state="translated">'"{0}" ist nicht Teil des Arbeitsbereichs.</target>

src/Workspaces/Core/Portable/xlf/WorkspacesResources.es.xlf

@@ -1317,6 +1317,11 @@ Las aserciones de búsqueda retrasada (lookbehind) positivas de ancho cero se us
         <target state="translated">El área de trabajo no está vacía.</target>
         <note />
       </trans-unit>
+      <trans-unit id="_0_is_in_a_different_project">
+        <source>{0} is in a different project.</source>
+        <target state="new">{0} is in a different project.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="_0_is_not_part_of_the_workspace">
         <source>'{0}' is not part of the workspace.</source>
         <target state="translated">'{0}' no es parte del área de trabajo.</target>

src/Workspaces/Core/Portable/xlf/WorkspacesResources.fr.xlf

@@ -1317,6 +1317,11 @@ Les assertions arrière positives de largeur nulle sont généralement utilisée
         <target state="translated">L'espace de travail n'est pas vide.</target>
         <note />
       </trans-unit>
+      <trans-unit id="_0_is_in_a_different_project">
+        <source>{0} is in a different project.</source>
+        <target state="new">{0} is in a different project.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="_0_is_not_part_of_the_workspace">
         <source>'{0}' is not part of the workspace.</source>
         <target state="translated">'{0}' ne fait pas partie de l'espace de travail.</target>

src/Workspaces/Core/Portable/xlf/WorkspacesResources.it.xlf

@@ -1317,6 +1317,11 @@ Le asserzioni lookbehind positive di larghezza zero vengono usate in genere all'
         <target state="translated">L'area di lavoro non è vuota.</target>
         <note />
       </trans-unit>
+      <trans-unit id="_0_is_in_a_different_project">
+        <source>{0} is in a different project.</source>
+        <target state="new">{0} is in a different project.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="_0_is_not_part_of_the_workspace">
         <source>'{0}' is not part of the workspace.</source>
         <target state="translated">'{0}' non fa parte dell'area di lavoro.</target>

src/Workspaces/Core/Portable/xlf/WorkspacesResources.ja.xlf

@@ -1317,6 +1317,11 @@ Zero-width positive lookbehind assertions are typically used at the beginning of
         <target state="translated">ワークスペースが空ではありません。</target>
         <note />
       </trans-unit>
+      <trans-unit id="_0_is_in_a_different_project">
+        <source>{0} is in a different project.</source>
+        <target state="new">{0} is in a different project.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="_0_is_not_part_of_the_workspace">
         <source>'{0}' is not part of the workspace.</source>
         <target state="translated">'{0}' はワークスペースの一部ではありません。</target>

src/Workspaces/Core/Portable/xlf/WorkspacesResources.ko.xlf

@@ -1317,6 +1317,11 @@ Zero-width positive lookbehind assertions are typically used at the beginning of
         <target state="translated">작업 영역이 비어 있지 않습니다.</target>
         <note />
       </trans-unit>
+      <trans-unit id="_0_is_in_a_different_project">
+        <source>{0} is in a different project.</source>
+        <target state="new">{0} is in a different project.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="_0_is_not_part_of_the_workspace">
         <source>'{0}' is not part of the workspace.</source>
         <target state="translated">'{0}'은(는) 작업 영역의 일부가 아닙니다.</target>

src/Workspaces/Core/Portable/xlf/WorkspacesResources.pl.xlf

@@ -1317,6 +1317,11 @@ Pozytywne asercje wsteczne o zerowej szerokości są zwykle używane na początk
         <target state="translated">Obszar roboczy nie jest pusty.</target>
         <note />
       </trans-unit>
+      <trans-unit id="_0_is_in_a_different_project">
+        <source>{0} is in a different project.</source>
+        <target state="new">{0} is in a different project.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="_0_is_not_part_of_the_workspace">
         <source>'{0}' is not part of the workspace.</source>
         <target state="translated">'Element „{0}” nie jest częścią obszaru roboczego.</target>

src/Workspaces/Core/Portable/xlf/WorkspacesResources.pt-BR.xlf

@@ -1317,6 +1317,11 @@ As declarações de lookbehind positivas de largura zero normalmente são usadas
         <target state="translated">Workspace não está vazio.</target>
         <note />
       </trans-unit>
+      <trans-unit id="_0_is_in_a_different_project">
+        <source>{0} is in a different project.</source>
+        <target state="new">{0} is in a different project.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="_0_is_not_part_of_the_workspace">
         <source>'{0}' is not part of the workspace.</source>
         <target state="translated">"{0}" não é parte do workspace.</target>

src/Workspaces/Core/Portable/xlf/WorkspacesResources.ru.xlf

@@ -1317,6 +1317,11 @@ Zero-width positive lookbehind assertions are typically used at the beginning of
         <target state="translated">Рабочая область не пуста.</target>
         <note />
       </trans-unit>
+      <trans-unit id="_0_is_in_a_different_project">
+        <source>{0} is in a different project.</source>
+        <target state="new">{0} is in a different project.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="_0_is_not_part_of_the_workspace">
         <source>'{0}' is not part of the workspace.</source>
         <target state="translated">'"{0}" не является частью рабочей области.</target>

src/Workspaces/Core/Portable/xlf/WorkspacesResources.tr.xlf

@@ -1317,6 +1317,11 @@ Sıfır genişlikli pozitif geri yönlü onaylamalar genellikle normal ifadeleri
         <target state="translated">Çalışma alanı boş değil.</target>
         <note />
       </trans-unit>
+      <trans-unit id="_0_is_in_a_different_project">
+        <source>{0} is in a different project.</source>
+        <target state="new">{0} is in a different project.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="_0_is_not_part_of_the_workspace">
         <source>'{0}' is not part of the workspace.</source>
         <target state="translated">'{0}' çalışma alanının parçası değildir.</target>

src/Workspaces/Core/Portable/xlf/WorkspacesResources.zh-Hans.xlf

@@ -1317,6 +1317,11 @@ Zero-width positive lookbehind assertions are typically used at the beginning of
         <target state="translated">工作区不为空。</target>
         <note />
       </trans-unit>
+      <trans-unit id="_0_is_in_a_different_project">
+        <source>{0} is in a different project.</source>
+        <target state="new">{0} is in a different project.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="_0_is_not_part_of_the_workspace">
         <source>'{0}' is not part of the workspace.</source>
         <target state="translated">'“{0}”不是工作区的一部分。</target>

src/Workspaces/Core/Portable/xlf/WorkspacesResources.zh-Hant.xlf

@@ -1317,6 +1317,11 @@ Zero-width positive lookbehind assertions are typically used at the beginning of
         <target state="translated">工作區不是空的。</target>
         <note />
       </trans-unit>
+      <trans-unit id="_0_is_in_a_different_project">
+        <source>{0} is in a different project.</source>
+        <target state="new">{0} is in a different project.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="_0_is_not_part_of_the_workspace">
         <source>'{0}' is not part of the workspace.</source>
         <target state="translated">'{0}' 不是工作區的一部分。</target>

src/Workspaces/CoreTest/SolutionTests/SolutionTests.cs

@@ -241,6 +241,24 @@ public void RemoveTwoDocumentsFromDifferentProjects()
             Assert.All(solution.Projects, p => Assert.Empty(p.Documents));
         }
 
+        [Fact, Trait(Traits.Feature, Traits.Features.Workspace)]
+        public void RemoveDocumentFromUnrelatedProject()
+        {
+            var projectId1 = ProjectId.CreateNewId();
+            var projectId2 = ProjectId.CreateNewId();
+
+            var documentInfo1 = DocumentInfo.Create(DocumentId.CreateNewId(projectId1), "file1.cs");
+
+            var solution = CreateSolution()
+                .AddProject(projectId1, "project1", "project1.dll", LanguageNames.CSharp)
+                .AddProject(projectId2, "project2", "project2.dll", LanguageNames.CSharp)
+                .AddDocument(documentInfo1);
+
+            // This should throw if we're removing one document from the wrong project. Right now we don't test the RemoveDocument
+            // API due to https://github.com/dotnet/roslyn/issues/41211.
+            Assert.Throws<ArgumentException>(() => solution.GetProject(projectId2).RemoveDocuments(ImmutableArray.Create(documentInfo1.Id)));
+        }
+
         [Fact, Trait(Traits.Feature, Traits.Features.Workspace)]
         public async Task TestOneCSharpProjectAsync()
         {